of the Aromatic Nucleus. 405 



of this plane is a system of five carbon and five hydrogen 

 atoms. From this inequality it follows that the valencies of 

 the carbon atom a, and similarly of any other of the six 

 carbon atoms, will be unsymmetrically directed. The direc- 

 tions of the valencies of any particular carbon atom are 

 determined, not by the symmetry of the whole molecule about 

 its centre, but by the configuration and mode of attachment 

 of the rest of the molecule about the atom. Such a view is 

 not inconsistent with any of the facts which support the 

 theory of Van't Hoff. 



Armstrong has stated (J. C. S. 1888) that in " the symbolic 

 system introduced by Yan't Hoff a double bond is represented 

 as the precise equivalent of two, and a treble bond as that of 

 three single bonds ; which all observations show is a mis- 

 representation of the facts." This appears to me to be a 

 misconception, for I have always considered the instability of 

 " unsaturated compounds " to be dependent upon the fact 

 that the forces of chemical affinity between two " doubly- 

 linked " carbon atoms are not exerted in the imaginary 

 straight line joining the atoms, but have to act, as it were, 

 round a corner. Their effective value is weakened in accord- 

 ance with the laws of the resolution of forces. 



It has also been stated that the formula of Thomsen is im- 

 possible, because it represents a system of atoms which 

 could not possibly be in equilibrium. This assertion involves 

 the assumption that the forces which bind the atoms together 

 act only in the directions ab, bc,cd,de, ef, fa (taken in order) 

 and along ad, be, and cf. It is in direct opposition to New- 

 ton's third law of motion. Each atom offers resistance to the 

 interpenetration of its sphere of action by that of another 

 atom. The force necessary to compress a liquid proves that 

 this resistance exists in the case of molecules. This point 

 may be illustrated by the fact that a model of Thomsen's 

 symbol will hold rigidly together if made of six equal spheres, 

 of which the centres are connected by flexible and inexten- 

 sible strings a b, be, c d, de, ef, fa, ad, b e, and ef in such 

 a manner that the strings a b to fa inclusive are each equal 

 in length to the diameters of the spheres, whilst ad, be, and cf 

 are each equal to this length multiplied by V2. (See fig. 1.) 

 The supposition that the spheres of action of the carbon 

 atoms in benzene are so related is the only one consistent 

 with the view that the atoms approach one another as nearly 

 as possible. If the longer strings be cut it will be found 

 possible to open out the model so that the centres of the 

 spheres form the angular points of a plane hexagon. I do 



